This invention relates to an OFDM receiving method and apparatus. More particularly, the invention relates to an OFDM receiving method and apparatus for receiving a signal that has sustained inter-symbol interference owing to delayed waves that exceed a guard interval, eliminating at least inter-symbol interference from the receive signal and applying demodulation and decoding processing to the receive signal from which interference has been eliminated.
In wideband wireless communications, frequency-selective fading ascribable to a multipath environment occurs. A useful scheme for dealing with this is multicarrier modulation, which divides the transmission band into a plurality of narrow bands (subcarriers) and transmits them in parallel in such a manner that frequency-selective fading will not occur. At present, specifications relating to digital TV/audio broadcasting (Japan and Europe) and wireless LAN (IEEE 802.11a) are being standardized with the Orthogonal Frequency Division Multiplex (OFDM) transmission scheme, which is one type of multicarrier modulation scheme, as the base. Further, OFDM-based modulation schemes have been proposed even in next-generation mobile communication systems.
In OFDM transmissions, a guard interval GI is inserted in order to eliminate inter-symbol interference (ISI) ascribable to multipath. FIG. 8 is a diagram useful in describing insertion of a guard interval. If M subcarrier samples (=1 OFDM symbol) are taken as one unit, then GI insertion signifies copying the tail-end portion of this symbol to the leading-end portion thereof. Inserting a guard interval GI makes it possible to eliminate the effects of inter-symbol interference ascribable to multipath.
FIG. 9 is a diagram useful in describing inter-symbol interference due to delayed waves, where A represents a direct wave and B a delayed wave (reflected wave). If delay time τ of delayed wave B is less than a guard-interval length NG, as indicated at (a) in FIG. 9, then a data symbol D0 of direct wave A will not overlap other data symbols of delayed wave B in a window timing W and inter-symbol interference will not occur. However, if delay time τ of delayed wave B is greater than the guard-interval length NG, as indicated at (b) in FIG. 9, data symbol D0 of direct wave A will overlap another data symbol D−1 of delayed wave B in the window timing W and inter-symbol interference will occur. Accordingly, maximum delay time τmax, of the delayed wave is taken into account and the guard-interval length NG is decided in such a manner that ISI will not occur.
In view of the foregoing, even if multipath delayed waves within the length of a GI symbol exist in an OFDM transmission, ISI will not occur owing to insertion of the GI. An advantage of this is that decoding can be performed without using equalization (i.e., the transmission is resistant to multipath fading). On the other hand, the adding on of a GI symbol (redundant symbol) causes a decline in transmission efficiency; the greater the GI symbol length, the more transmission efficiency declines. Accordingly, the present applicant has proposed a method whereby ISI and inter-carrier interference (ICI) can be reduced even when the GI symbol length is not made greater than the maximum delay time τmax of the delayed wave (see Japanese Patent Application Laid-Open Nos. 2004-208254 and 2004-289475).
FIG. 10 is a diagram useful in describing ISI elimination. A delayed wave B lags behind a direct wave A by the GI length NG or more, and the delay time Nτmax satisfies the relation NG<Nτmax. The data symbol D0 of the direct wave A partially overlaps a pilot symbol P of the delayed wave B and sustains ISI from the pilot symbol P of the delayed wave B. It is necessary, therefore, to remove this portion [see the hatched portion at (d) of FIG. 10] of the pilot symbol from the receive signal. The time over which interference is received (the number of samples) is y=(Nτmax−NG). Accordingly, the y portion is cut out of the already known pilot-signal waveform and is generated as an ISI replica RPP [see the hatched portion at (e) of FIG. 10]. The ISI replica RPP is subtracted from the receive signal [see the left side at (f) of FIG. 10]. As a result, the portion of the delayed wave that interferes with the direct wave can be made zero and it is possible to eliminate ISI interference. That is, by applying FFT processing to the signal at (f) of FIG. 10, OFDM demodulation that will not be affected by ISI becomes possible.
In a manner similar to that set forth above, a data symbol D1 (see FIG. 10) of the direct wave A partially overlaps the preceding data symbol D0 of the delayed wave B and sustains ISI interference from the data symbol D0 of delayed wave B. It is necessary, therefore, to remove this portion of the data symbol D0 from the receive signal. The time over which ISI interference is received (the number of samples) is y. Accordingly, the y portion is cut out of the time waveform of the preceding data symbol D0 and is generated as an ISI replica RP0 [see the hatched portion at (e) of FIG. 10]. The ISI replica RP0 is subtracted from the receive signal [see the right side at (f) of FIG. 10]. As a result, the portion of the delayed wave that interferes with the direct wave can be made zero and it is possible to eliminate ISI interference. That is, by applying FFT processing to the signal at (f) of FIG. 10, OFDM demodulation that will not be affected by ISI becomes possible.
FIG. 11 is a diagram useful in describing elimination of ICI. By virtue of the ISI elimination processing described above, the portion of the ISI segment y of the delayed wave B shown at (a) of FIG. 11 is eliminated, thereby generating the receive signal indicated at (b), and this signal is subjected to FFT processing to eliminate inter-symbol interference ISI. However, the delayed wave B becomes discontinuous at the portion of the ISI segment y. Moreover, the waveform is no longer periodic. Consequently, each subcarrier component obtained by FFT processing includes distortion ascribable to inter-carrier interference ICI. In order to eliminate such inter-carrier interference ICI, it is necessary to insert a waveform so as to smoothen the ISI segment y of the delayed waveform B and make the delayed waveform periodic, as indicated by the dashed line at (c) of FIG. 11.
Accordingly, the receive signal at (b) of FIG. 11 is subjected to FFT processing, as shown at (d) of FIG. 11, after which IFFT processing is applied, thereby providing a continuous signal waveform, as shown at (e) of FIG. 11. If the tail-end segment y of the continuous signal waveform at (e) of FIG. 11 is cut out and inserted into the segment y at the front end of the receive signal at (b) of FIG. 11, the delayed wave B will become a continuous periodic waveform, as shown at (f) of FIG. 11. If FFT processing is applied to the signal at (f) of FIG. 11, ICI is suppressed. It should be noted that, ideally, it is necessary for the delayed wave B shown at (f) of FIG. 11 to be made a continuous periodic waveform by the waveform at (e) of FIG. 11. However, since the waveform at (b) of FIG. 11 is subjected to FTT and IFFT processing, the waveform at (e) of FIG. 11 departs slightly from the ideal shape.
FIG. 12 is a block diagram of an OFDM receiver for applying OFDM demodulation processing to a signal that has undergone ISI and ICI removal, subjecting the demodulated signal obtained to decoding processing and outputting the decoded signal.
A GI removing unit 100 removes the GI from the receive signal, and a pilot/data separating unit 101 separates data and a pilot from the receive signal, inputs the data to an ISI replica generating/eliminating unit 102 and inputs the pilot to a delay profile generator 103. The delay profile generator 103 calculates the correlation between the receive pilot signal and the already known pilot signal and outputs a delay profile.
An ISI replica generator 102a adopts, from the delay profile, a delay-time portion greater than the guard-interval-length NG as an ISI portion and generates, as an ISI replica, a time-waveform portion of the already known symbol (pilot symbol), which conforms to the ISI portion, or a time-waveform portion of the preceding symbol [see (e) of FIG. 10]. A channel compensator 102b multiplies the ISI replica by a channel estimation value, thereby applying channel compensation, and inputs the result to a subtractor 102c. The latter subtracts the ISI replica from the receive signal and inputs the direct wave A and delayed wave B (data symbol D0), which are shown on the left side at (f) of FIG. 10, to an FFT processor 104, which is the next stage.
The FFT processor 104 applies FFT processing to the receive signal input thereto, thereby generating data elements on a per-subcarrier basis. An FFT processor 105 applies FFT processing to the channel estimation value, thereby generating a channel compensation value for each subcarrier. A channel compensator 106 multiplies the result of FFT processing by a channel compensation value subcarrier by subcarrier, demodulates data elements of the number of subcarriers that constitute the data symbol D0 and inputs the result to a replica generating/ICI eliminating unit 107.
The replica generating/ICI eliminating unit 107 has an IFFT processor 107a which applies IFFT processing to the demodulated data of the number of subcarriers constituting the data symbol D0 output from the channel compensator 106 and outputs the time-waveform signal [see (e) of FIG. 11] of data symbol D0. A demodulated-signal restoration replica generator 107b cuts out the signal portion of the y segment [see (e) of FIG. 11] at the tail end of the time-waveform signal that enters from the IFFT processor 107a, thereby generating a demodulated-signal restoration replica (ICI replica), and inputs the replica to a combiner 107c. The latter adds the demodulated-signal restoration replica to the receive signal [see (d) of FIG. 11] that is output from the subtractor 102c, thereby producing a continuous signal waveform, and inputs this waveform to an FFT processor 108a of an OFDM demodulator 108.
The FFT arithmetic unit 108a applies FFT processing to the combined signal that is output from the combiner 107c, thereby generating data elements on a per-subcarrier basis. A channel compensator 108b multiplies the result of FFT processing by the channel compensation value subcarrier-by-subcarrier and outputs the result of channel estimation as a decoded signal. As a result of the operation described above, inter-carrier interference ICI can be suppressed together with inter-symbol interference ISI.
A decoding processor 109 applies error-correcting decoding processing to OFDM-demodulated data (soft-decision data) and outputs the decoded result (hard-decision data). Examples of an error-correcting code that can be used are a turbo code, which is capable of implementing an error characteristic near the Shannon limit, and a low-density parity check (LDPC: Low Density Parity Check) code. A method of improving the error characteristic by updating a priori probability of each receive bit stream of a code word, which has been obtained prior to decoding processing, to a value having a high probability through iterative decoding is used as the decoding method in these error-correcting codes.
An error-correcting encoder 110 encodes the result of decoding processing, and an IFFT processor 111 applies IFFT processing to the decoded data of the number of subcarriers that constitute the encoded data symbol D0 and outputs the time-waveform signal of the data symbol D0. A delay circuit 112 delays this time-waveform signal by a time equivalent to one symbol time TS and inputs the delayed signal to the ISI replica generator 102a. 
In a manner similar to that described above, a data symbol D1 (see FIG. 10) of the direct wave A partially overlaps the preceding data symbol D0 of the delayed wave B and sustains ISI from the data symbol D0 of the delayed wave B. It is necessary, therefore, to remove this portion of the data symbol D0 from the receive signal. The time (number of samples) subjected to ISI is y. Accordingly, the ISI replica generator 102a cuts the y portion out of the time-waveform signal (the output signal of the delay circuit 112) of the previous data symbol D0 to generate it as the ISI replica [see the hatched portion at (e) of FIG. 10]. The channel compensator 102b multiplies the ISI replica by the channel estimation value, thereby applying channel compensation, and inputs the result to the subtractor 102c. The latter subtracts the ISI replica from the receive signal and inputs the direct wave A and delayed wave B (data symbol D1), which are shown on the right side at (f) of FIG. 10, to the FFT processor 104, which is the next stage. Thenceforth, and in similar fashion, processing is executed in a manner similar to that of data symbol D0.
In accordance with the OFDM receiver described above, the BER (Bit Error Rate) characteristic can be improved by eliminating ISI and ICI. Further, by repeating a series of processes (referred to as “turbo equalization processing”), namely replica generation, ISI and ICI removal, demodulation and decoding, an excellent transmission characteristic can be implemented even in a multipath environment in which the GI length is exceeded. In addition, if code is a turbo code and LDPC code, the greater the number of decoding iterations, the more the error characteristic can be improved.
Thus, the greater the number α of turbo equalization iterations and number β of decoding iterations, the better the reception characteristic. A problem, however, is that the greater the numbers α, β of iterations, the longer the processing time required. In application to an actual system, therefore, it is necessary to limit the number of turbo equalization iterations and number of decoding iterations in such a manner that receive processing will be completed within a stipulated period of time.
Several combinations of numbers of turbo equalization iterations and numbers of decoding iterations for completing receive processing within the stipulated time are conceivable. Which combination will give the best reception characteristic differs depending upon the propagation path. In a mobile communication environment, the propagation path changes with time. Consequently, in a case where receive processing has been executed with a combination of predetermined numbers of turbo equalization iterations and decoding iterations, a problem which arises is that the optimum reception characteristic cannot always be obtained.
Further, in a case where the combination of number of turbo equalization iterations and number of decoding iterations is changed, a function and criteria for estimating the propagation path and deciding the combination of number of turbo equalization iterations and number of decoding iterations using the estimated propagation path are necessary.